The invention concerns a method of magnetic resonance imaging with which the spins are excited in an investigation volume using radio frequency pulses and the signal produced thereby is subsequently read out following a short echo time te following appropriate spatial encoding, wherein repetition of the sequence for spatial encoding is carried out in a time interval tr with appropriate variation of the spatial encoding gradients.
A method of this type is known in the art from the publication of Haase, Magnetic Resonance in Medicine 13, pages 77-89 (1990).
This type of method facilitates magnetic resonance imaging of pulsatile moving blood-vessels e.g. coronary arteries. A problem related to the imaging of these objects is the fact that the allowable measuring time for the observation is extremely short and must be substantially below the time of an EKG cycle in order to avoid artifacts due to changes in the signal intensity in consequence of pulsatile flow as well as due to pulsatile motion of the vessel. The imaging of twisted or helical-shaped vessels such as the coronary arteries has additional complications using a slice-selective method since the vessel moves out of the observation slice in a manner which is very difficult to predict.
In conventional MR-angiography, imaging is attempted with the assistance of a multi-slice method, wherein a plurality of phase encoding steps are recorded per excitation step in an EKG cycle so that the entire recording process can be carried out within a time interval of up to 30 s in the absence of breathing. With the assistance of this technology it is possible to sectionwise image at least the thicker sections at the beginning of the coronary arteries when the orientation of the vessel section to be observed coincides with the selected slice. A method of this kind is known in the art due to Edelman RR, Radiology 181:641 (1991).
An alternative means for the avoidance of artifacts due to breathing motion is the so-called Navigator Echo Technique (Wang Y., Magnetic Resonance in Medicine 33:713 (1995)). In both procedures additional fat suppression pulses can be used to improve the signal of the vessels relative to the environment. Both utilize slice selective data collection and allow for imaging of the vessel in a sectionwise manner if required.
The problem of spatial recording of geometrically complicated vessels can in principal be solved with the assistance of three-dimensional data recording methods and subsequent reconstruction in accordance with the maximum intensity algorithm. Complex vessel dependencies can also be properly imaged using bolus injections of contrast media and very fast gradient echo methods leading to a very high signal for blood in vessels. A method of this kind is known in the art through the publication of Debatin, Proceedings 4th Meeting ISMRM, 1966, page 161. However, imaging requires an amount of time which is long compared to a heart cycle. The moving heart and the coronary vessels can therefore only be imaged in a defocussed manner using such a method or not at all.
Another possibility for imaging spatially complex vessels is given by the projection method. Such methods were formerly proposed for MR angiography and are e.g. known in the art from the publication of Nishimura so (Magnetic Resonance in Medicine 1991 January; 17(1):126-40). However, projection techniques have very large interference in comparison to the slice-selective method or in comparison to three-dimensional techniques from incomplete suppression of signals from stationary tissue due to the effective large thickness of the observed slice. Due to this fact techniques of this type are no longer used in MR angiography. Rather a desired view of a two-dimensional projection through a thick three-dimensional slice is effected by post-processing of three-dimensional recordings with the assistance of the so-called maximum intensity projection (MIP) algorithm.
It is therefore the purpose of the present invention to present a method for time-resolved MR angiography which is capable of recording projection angiograms in a measuring time of less than 1 s.